1. Field of the Invention
The present invention relates to circuit board technology.
2. Background Art
A transmission line is a pair of electrical conductors on a circuit board used to carry an electrical signal and corresponding reference signal. The distribution of a magnetic field within and around a transmission line determines the amount of self-inductance of the transmission line and the mutual-inductance of the transmission line to adjacent signal lines. These inductance values are major factors in defining transmission line characteristics such as crosstalk and characteristic impedance. The amplitude and timing integrity of the signals carried by a transmission line are highly dependent on these two characteristics. The magnetic field distribution of a transmission line is a significant factor in preserving signal integrity, and thus is an important factor in the intended function of a transmission line.
In one configuration, a transmission line may include a signal trace and a reference plane. The signal trace carries the forward electrical signal of the transmission line pair, and the reference plane carries the return current related to the electrical signal. The reference plane may be a ground plane or other voltage plane of a circuit board. The signal trace is typically routed over the reference plane. A layer of a dielectric material separates the signal trace and the reference plane. A net magnetic field around such a transmission line is a vectorial sum of a first magnetic field due to the forward current on the signal trace and a second magnetic field due to the return current on the reference plane. When the return current path on the reference plane is very close in distance and size to the forward current path, the first and second magnetic field components cancel out each other at most locations around the transmission line, due to their opposing phases. In such a case, the net magnetic field tends to be mainly concentrated between the signal trace and the reference plane.
A reference layer may be connected to a ground or voltage reference potential, and may be located above or below one or more signal traces of the circuit board. Such a reference layer configuration aids in controlling a characteristic impedance of transmission lines, a crosstalk between adjacent signal traces, and an electromagnetic interference (EMI) radiation emanating from transmission lines, by confining the electric and magnetic fields in a smaller region around the transmission lines. However, implementing one or more reference planes in a circuit board leads to an increase in the number of metal layers in the circuit board. The increase in the number of metal layers increases a cost and a thickness of the circuit board. Such cost and thickness increases are not desired, especially for consumer electronics devices and systems that need to be as thin as possible and have dwindling average selling prices.